1. Field of the Invention
The invention relates generally to knee prostheses and, more specifically, to knee prostheses which include the use of posterior stabilized inserts and have an extended range of flexion.
2. General Background of the Invention
Arthritis of the knee is a disease in which the surfaces of the knee gradually “wear out.” Osteoarthritis, characterized by chronic degeneration of the cartilage of the joints, is the most common form of arthritis. This may be due to either old age, congenital deformity, or damage due to trauma. Osteoarthritis, characterized by chronic degeneration of the cartilage of the joints, is the most common form of arthritis. Systemic arthritis, such as rheumatoid arthritis, or gout affects the synovium (the membrane tissue in the joint that normally lubricates the joint), becomes pathologic and the surface of the joint is destroyed. In either case, when the surface of the joint is worn away, the activities of daily living can become very difficult. Standardized treatment such as weight loss, anti-inflammatory medication, braces, orthotics, steroid injections, physical therapy may be effective.
In many cases, however, despite the above non-surgical treatments, functional limitations persist. In such cases, disease and trauma affecting the articular surfaces of the knee joint are commonly treated by surgically replacing the ends of the femur and tibia with prosthetic femoral and tibial implants, referred to as total knee replacement (TKR).
In TKR surgery, a surgeon typically affixes two prosthetic components to the patient's bone structure; a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component respectively.
The femoral component is placed on a patient's distal femur after appropriate resection of the femur. The femoral component is usually metallic, having a highly polished outer condylar articulating surface, which is commonly J-shaped.
A common type of tibial component uses a tray or plateau that generally conforms to the patient's resected proximal tibia. The tibial component also usually includes a stem that extends at an angle to the plateau in order to extend into a surgically formed opening in the patient's intramedullary canal. The tibial component and tibial stem are both usually metallic.
A plastic or polymeric (often ultra high molecular weight polyethylene) insert or bearing fits between the tray of the tibial component and the femoral component. This tibial insert provides a surface against which the femoral component condylar portion articulates, i.e., moves in gross motion corresponding generally to the motion of the femur relative to the tibia.
In some knee prostheses, the insert also engages in motion relative to the tibial tray. Such motion can be translational and/or rotational sliding motion relative to the tibial plateau. In other types of knee prostheses with tibial inserts, the tibial inserts can engage in other types of motion relative to the tibial plateau and/or femoral component.
Modern TKR's are tricompartmental designs; they replace three separate articulating surfaces within the knee joint: the patello-femoral compartment and the lateral and medial inferior tibio-femoral compartments. Most TKR's are designed to articulate from a position of slight hyperextension to approximately 115 to 130° flexion. A tricompartmental design can meet the needs of most TKR patients even though the healthy human knee is capable of a range of motion (ROM) approaching 170°. However, there are some TKR patients who have a particular need to obtain high flexion in the knee joint. For many, a TKR that permits patients to achieve a ROM in excess of 130° is desirable to allow deep kneeling, squatting and sitting on the floor with the legs tucked underneath.
Another problem encountered by TKR patients is unwanted movement of the femoral component on the tibial component. This occurs when the ligaments of the knee are “tight,” or not tensioned properly, during the TKR procedure. Ligaments located on the side of the knee where the deformity is present become tight due to contraction of the compartment. A tight posterior cruciate ligament may cause the knee to move in an unnatural motion. A posterior stabilized insert may assist in preventing the femoral component from unnatural motion on the tibial component by providing posterior support after the posterior cruciate ligament is removed. However, current inserts providing posterior support are designed to allow a ROM to only about 120°. When a patient with a standard posterior support insert demands deeper flexion, the proximal edge of the femoral condyle edge loads into the posterior edge of the insert. This can lead to excessive polyethylene wear. The extreme posterior location of the contact point may also lead to lateral condylar subluxation as the tibia internally rotates. Deeper flexion also leads to increased femoral translation to the posterior edge of the insert. In conforming knee designs, this can limit range of motion because more implant material is located on the posterior edge of the insert. As the femoral component engages the thicker part of the insert, the lateral and medial collateral ligaments reach their strain limit, thus preventing further posterior translation and limiting flexion by impinging the posterior edge of the insert against the posterior cortex of the femur. Thus, there is a need for an insert that provides sufficient posterior support and reduces posterior conformity. Also needed is an insert that allows a ROM beyond 120° and minimizes polyethylene wear and accommodates condylar rotation.